1. Field of the Invention
The present invention relates to a power transmission device utilized in the form of a toroidal type continuously variable transmission employed as, e.g., a transmission for an automobile.
2. Related Background Art
There has been pursued the use of a toroidal type continuously variable transmission illustrated in FIGS. 3A and 3B as a transmission for an automobile. In this toroidal type continuously variable transmission, as disclosed in, e.g., Japanese Utility Model Laid-Open Application No. 62-71465, an input-side disk 2 is supported concentrically with an input shaft 1, and an output-side disk 4 is fixed to the end of an output shaft 3. A casing encases the toroidal type continuously variable transmission. A support bracket is provided on the inner surface of the casing or inwardly of this casing. The support bracket is provided with trunnions 5, 5 swinging about a twist position with respect to the input shaft 1 and the output shaft 3.
Each of the trunnions 5, 5 is composed of a metal material exhibiting a sufficient rigidity. Power rollers 7, 7 are rotatably supported on the peripheries of displacement shafts 6, 6 provided at the central parts of the respective trunnions 5, 5. Each of the power rollers 7, 7 is interposed between the input- and output-side disks 2, 4. An input-side concave surface 2a and an output-side concave surface 4a are formed on side surfaces, opposite to each other, of the input- and output-side disks 2, 4 in the axial direction. The concave surfaces 2a, 4a assume circular arcs in section that are formed about the swinging centers of the respective trunnions 5, 5. Then, the input- and ouput-side concave surfaces 2a, 4a are brought into contact with outer peripheral surfaces 7a, 7a of the power rollers 7, 7 that are formed as spherically convex surfaces.
A loading cam type pressure unit 8 is interposed between the input shaft 1 and the input-side disk 2. This pressure unit 8 elastically thrusts the input-side disk 2 toward the output-side disk 4. The pressure unit 8 is constructed of a cam plate 9 rotating together with the input shaft 1 and a plurality of (e.g., four) rollers, 11 held by a retainer 10. A cam surface 12 having concave and convex portions concaved and convexed in the peripheral direction is formed on one surface (right surface in FIGS. 3A and 3B) of the cam plate 9. Further, a similar cam surface 13 is formed on an external surface (left surface in FIGS. 3A and 3B) of the input-side disk 2. Then, the plurality of rollers 11 are rotatable about axes in the radial directions with the input shaft 1 being centered.
When the cam plate 9 rotates with rotation of the input shaft 1, the cam surface 12 presses the plurality of rollers 11 against the cam surface 13 formed on the outer end surface of the input-side disk 2. As a result, the input-side disk 2 is pressed against the plurality of power rollers 7, 7. Simultaneously, the input-side disk 2 rotates when a pair of the cam surfaces 12, 13 engage with the plurality of rollers 11. Subsequently, the rotations of this input-side disk 2 are transmitted via the plurality of power rollers 7, 7 to the output-side disk 4. The output shaft 3 fixed to the output-side disk 4 rotates.
FIG. 3A shows a state of the transmission in which the trunnions 5, 5 are swung to effect a deceleration. In this state, the displacement shafts 6, 6 are tilted so that the outer peripheral surfaces 7a, 7a of the power rollers 7, 7 are each brought into contact with a closer-to-center part of the input-side concave surface 2a and with a closer-to-outer-periphery part of the output-side concave surface 4a. Conversely, when effecting an acceleration, the trunnions 5, 5 are swung as shown in FIG. 3B. The displacement shafts 6, 6 are tilted so that the outer peripheral surfaces 7a, 7a of the power rollers 7, 7 contact with the closer-to-outer-periphery part of the input-side concave surface 2a and with the closer-to-center part of the output-side concave surface 4a. Tilt angles of the displacement shafts 6, 6 are set intermediate between those illustrated in FIGS. 3A and 3B, thereby obtaining an intermediate transmission gear ratio between the input shaft 1 and the output shaft 3.
For enhancing the driving force transmittable by the above-mentioned toroidal type continuously variable transmission, it has hitherto been preferable to increase contact surface pressures on contact areas through which the power is transmitted, i.e., the contact areas between the input-side concave surface 2a and the outer peripheral surface 7a, 7a of the power rollers 7, 7 in the case of the toroidal type continuously variable transmission illustrated in, e.g., FIGS. 3A and 3B.
When simply increasing the contact surface pressure on each contact surface area, however, a decrease in the life-span of rolling fatigue occurs in the contact surface areas due to a repetitive stress applied onto the rolling surface. Also, there arises a problem in which a loss in the power transmission on each contact surface area increases. This leads to an increase in the power required for rotating the input-side disk 2.